7. Project Summary Misrepair of DNA damage is a hallmark of cancer. We discovered that the budding yeast Shu complex is a conserved regulator of DNA repair through a central role in Rad51 regulation. Rad51 functions during the high fidelity homologous recombination pathway to find and invade a homologous template for repair and also during replication fork protection and restart. Rad51 is tightly regulated in cells by accessory proteins, collectively called the Rad51 mediators, including the Shu complex. In humans, misregulation of hRAD51 or its mediators is associated with cancer predisposition (particularly breast and ovarian cancers) and Fanconi anemia, which is also characterized by anemia and cancer. We found that disruption of the yeast Shu complex leads to cellular death specifically upon exposure to alkylation induced DNA damage. Alkylation damage is caused by a myriad of industrial and consumer-based sources and is pervasive in our environment. DNA alkylation leads to replication stress and DNA damage. If DNA is alkylated during replication, then the replication fork can stall or collapse, and many repair mechanisms can be utilized to tolerate, bypass, or repair the damaged DNA. How a cell commits to a specific repair pathway is largely known. In budding yeast, the Shu complex is critical in the processing of replication forks damaged by alkylating agents. This complex is highly conserved throughout eukaryotes and contains the Rad51 paralogs, proteins that are structurally similar to the central DNA repair protein Rad51 and are mutated in cancer. In this study, we aim to elucidate the role of the yeast and human Shu complexes in repair of DNA alkylation damage at a replication fork. We are testing the hypothesis that the Shu complex is a critical key regulator of DNA damage tolerance at a replication fork by specifically recognizing alkylation induced DNA damage to promote Rad51-mediated template switch and protect forks from double-strand break induction by AP endonucleases. Using what we learn in yeast to quickly and efficiently identify key substrates, residues, and protein targets, we will expand our studies into human cell lines where we will investigate the role of the human Shu complex in tolerance of alkylation damage. In addition, we will identify at risk individuals harboring mutations in these important genes that may be more sensitive to DNA alkylation damage and therefore susceptible to cancer. Collectively, these studies will provide key insights into the role of the Shu complex in tolerance of DNA alkylation damage and elucidate how this complex promotes error-free DNA repair to prevent genetic instability and cancer. !